Review of England and Wales
Monitoring Data for Which a National or International Standard Has Been
Set
DWI0855
March 2008
EXECUTIVE SUMMARY
The Drinking Water Regulations that incorporate the provisions of the
European Union (EU) Drinking Water Directive into the law of England
and Wales contain a list of substances for which there are numerical
standards. However, there is also a requirement to consider other
potential contaminants that might be present in drinking water. A
number of authorities and jurisdictions outside the EU have produced
guidelines or standards for substances that are not specifically listed
in the EU Directive. There may, therefore, be legitimate questions from
stakeholders on what is known about the presence of these substances in
UK drinking waters. In addition, it is important to assess the
information that is available should there be a suggestion that these
additional substances be considered in revisions of the Directive.
Additional substances were identified in the World Health Organisation
(WHO) Guidelines for Drinking Water Quality and in standards from the
USA, Canada, Australia, New Zealand and Japan. Many of these were
individual pesticides. These are fully covered by the pesticide
parameters in the EU Directive, which refers to all pesticides and sets
a precautionary maximum contaminant level. Searches of the peer
reviewed and grey literature were performed for data on the
concentrations of these substances in drinking water in all parts of
the world, but particularly in the UK. Research on and monitoring of
the chemicals of interest, commissioned or carried out by DWI, the
Environment Agency or other UK authorities and agencies was also
investigated for relevant data. Most of the larger water companies were
approached to determine whether they had carried out investigations or
monitoring for these substances. Not all companies were able to help
but some provided a considerable amount of information. The data that
were received usually reflected high risk sources.
Although there are no systematic data on the listed substances, the
data that are available are helpful and indicate that the majority of
substances, where they are present, occur at concentrations below the
health-based guidelines from WHO. Occasional detections above the
guideline values, but well below other health-based standards, appear
to be associated with a very small number of naturally occurring
inorganic substances in groundwaters.
In the third edition of its Guidelines, WHO introduced the concept of
drinking water safety plans (DWSPs), which is an approach now generally
accepted as best practice. This approach has been formally incorporated
into the drinking water regulations for England and Wales and is based
on hazard identification and risk assessment, followed by management
procedures to mitigate those risks. The risks and mitigating procedures
should also be prioritised to ensure that resources are directed where
they will have the greatest impact. This approach provides a framework
for managing risks from source to tap and helps to ensure that hazard
identification occurs for substances that are potentially of concern
for drinking water. This means that investigation of their presence in
drinking water sources and in drinking water itself is targeted to
where such substances are likely to be found and takes existing
barriers into account. By doing this it is possible to demonstrate that
the barriers are appropriate and functioning properly so that the need
for extensive chemical analysis of specific substances is significantly
reduced. This approach is not only more efficient in directing
resources where they will have most impact but it also means that water
quality can be assured over a much wider range of potential
contaminants than would otherwise be possible. Chemical monitoring of
the final water merely informs us that there is a problem, while the
DWSP approach is designed to ensure that controls are in place that
will prevent a problem. Chemical monitoring is a final check on the
presence and amount of chemicals; it does not control them.
Several of the listed substances are inorganic and most of these are
present naturally as constituents of water, although there may also be
anthropogenic sources that contribute to concentrations in water. Most
of these have been assessed by the Environment Agency as part of their
groundwater monitoring programme and are not of concern for drinking
water. A DWI-commissioned in-depth study on uranium in drinking water
identified a small number of samples, taken from private water supplies
that exceeded the current WHO provisional guideline value, but not the
USEPA standard. An early study, part of the Regional Heart Study,
indicated that molybdenum concentrations may exceed the WHO guideline
value in some areas. Since molybdenum is an essential element, it
requires a different approach to risk assessment and DWI has therefore
commissioned a study of molybdenum in drinking water in England and
Wales.
A substantial number of the listed substances are volatile chlorinated
organic molecules, most of which are used as solvents or result from
the breakdown of solvents. Experience has shown that these do not occur
at greater than trace concentrations in surface waters because they
volatilise to atmosphere. However, if they are spilt or discharged to
soil and are able to reach groundwater they may persist for a
considerable time. Tri- and tetrachloroethene, and carbon
tetrachloride, are incorporated into the regulations and, therefore,
all waters are monitored for the presence of these substances. Because
of chemical similarity it is possible that many other chlorinated
organics would also be detected by the same analysis (if based on gas
chromatographic separation with electron capture or full-scan mass
spectrometric detection) and therefore action could be taken if
detected concentrations were significant, i.e., close to health-based
guideline values.
A small number of fuel related compounds are also included on the list.
The odour threshold for all of these substances in water is very low
and they are almost invariably unacceptable to consumers at
considerably lower concentrations than the health-based values.
The additional non- or semi-volatile substances on the list include the
cyanotoxin microcystin-LR, DEHP, DEHA, dioxins, EDTA and NTA. This is a
diverse group of compounds but, within this group, dioxins have been
shown not to occur in drinking water. This is because of their low
water solubility and high potential to adsorb to particulate matter and
sediment, which is readily removed during drinking water treatment.
While cyanotoxins do occur in raw water, their potential presence can
be readily identified by the fact that they are only present at
significant concentrations in association with large blooms of
cyanobacteria or blue-green algae. In addition, they are readily
removed by a number of treatment processes. Monitoring for such
substances is
unhelpful and the guideline value primarily provides a benchmark for
assessing the efficiency of treatment.
DEHP is widely used as a plasticizer and there is clear evidence that
it does reach drinking water at low concentrations, which are well
below the WHO guideline value.
EDTA and NTA are chelating agents that have been found at low
concentrations, well below WHO guidelines, in many waters receiving
treated wastewater. EDTA is a food additive and NTA is a detergent
builder. Both appear to be removed by advanced water treatment and
would not be expected to be present at concentrations of concern.
A number of disinfection by-products (DBPs) are included in the list.
These are primarily included in the WHO Guidelines as a benchmark
against which to assess the importance of such substances, which have
received a great deal of research and media attention. European
standards include a value for total trihalomethanes (TTHMs) while WHO
sets guideline values for individual substances. Two other
jurisdictions also include a total haloacetic acids value because
these, with THMs, form the dominant chlorination DBPs. The most
appropriate mechanism for controlling chlorination DBPs is the removal
of natural organic matter (NOM) with which chlorine reacts. This will
reduce all of the halogenated organic by-products while actually
helping to maintain effective disinfection. It also means that there is
no need to monitor a very wide range of by-products, the great majority
of which always occur at concentrations below guideline values or
health-based guidance concentrations. However, in some specific
situations, for example chlorination at low pH haloacetic acids (HAAs)
will increase even if THMs are reduced.
Several recommendations are made as a consequence of this study.
- Data on raw water sources of considerable interest with
regard to drinking water quality is collected by the Environment Agency
and DEFRA, either directly or through commissioned research. It is
recommended that lines of communication be established to ensure that
such data are made available to DWI to assist in judging the need for
any action and to allow DWI to respond to any legitimate questions
regarding contaminants/constituents of drinking water.
- The most important inorganic substances for which more data
were required are uranium and molybdenum. However, DWI has commissioned
work on both of these substances and the work on uranium has been
completed. Consideration should be given to whether it is possible to
carry out some specifically targeted investigations into the presence
of thallium, taking into account natural and industrial sources.
- It would be appropriate to target monitoring for additional
volatile chlorinated organic substances where tri-and tetrachloroethene
and carbon tetrachloride have been identified in anaerobic groundwater,
or where they have reached groundwater following passage through
anaerobic conditions.
- Data show that the health risks for drinking water
associated with microcystins from cyanobacteria in the UK are very low
and most water companies successfully manage drinking water sources to
minimise the numbers of cyanobacteria. However,
- The data on DEHP are relatively limited and it would be of
value to have a better view of the range of concentrations in drinking
water, which might be achievable by closer examination of water company
GC-MS scans.
- Although the data that exist indicate that EDTA and NTA are
present in raw and drinking water at concentrations well below the WHO
guidelines, it would be useful to obtain some modern data on their
occurrence in waters receiving significant inputs of wastewater.
However, this is not considered to be a high priority.
- When water companies are introducing processes to achieve
or assess reductions in THMs it would be appropriate also to consider
whether concentrations of HAAs have been reduced.
Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.